IC technologies using organic semiconductor devices have been drawing attention in recent years. Their major appeals are in that the IC can be manufactured at low costs and flexible resins can be used for a substrate. Because of those advantages, there are great expectations for application of organic semiconductor devices to a circuit that uses a plastic substrate, an electronic tag, a display driver circuit for a display, a memory, etc.
In general, an organic semiconductor device is composed of a substrate, a gate insulating layer, a gate electrode, a source electrode, a drain electrode, and an organic semiconductor layer, and is utilized as a thin film field effect transistor (FET) or the like.
Low molecular weight compounds such as polyacene, π-conjugated system polymer compounds such as polythiophene, and π-conjugated system olygomers are well-known examples of organic compounds that exhibit semiconductor characteristics and are used in devices.
In an organic FET, which is an FET having a semiconductor layer formed from an organic semiconductor, a change in voltage applied to a gate electrode (gate voltage Vg) causes an excess or shortage of electric charges at the interface between a gate insulating layer and an organic semiconductor layer. As a result, a drain-source current (Id), which flows from a source electrode to a drain electrode through the organic semiconductor layer, is changed to make it possible to perform switching.
When building an organic FET on a flexible resin substrate, a gate insulating film, an organic semiconductor layer, and other components on the substrate have to be formed at low temperature. This is to protect the resin substrate, which is weak against high temperature, from softening.
For instance, Bao et al. have proposed an organic TFT manufacturing method in which a polyimide film is formed by screen printing on a polyethylene terephthalate substrate where a gate electrode is previously formed from indium tin oxide, and the polyimide film is then baked at low temperature to obtain an insulating layer (see the article written by Z. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, and A. J. Lovinger on p. 1299 of Chem. Mater. 9, 1997).
To form a gate insulating layer by this method, the gate insulating layer has to be as thick as 200 nm or more. A thick gate insulating layer requires a large gate drive voltage (Vg) of several tens to hundred volts (V).
Japanese Patent Application Laid-Open No. 2002-110999 discloses a method of obtaining a gate insulating layer by spin coating of cyanoethyl pullulan dispersed with barium titanate on a resin substrate in order to reduce Vg. This gate insulating layer is 300 to 500 nm in thickness and is as thick as the one obtained by the method of Bao et al. However, this gate insulating layer which is formed at 120° C. (low temperature) and which is high in dielectric constant makes it possible to drive the resultant organic FET with low voltage. On the other hand, in this type of insulating film, an amorphous compound that serves as the base material (cyanoethyl pullulan in this case) is not completely densified because of the low temperature heat treatment, leaving numerous minute voids in the gate insulating layer. The voids inhibit inorganic compound particles dispersed (barium titanate in this case) from exhibiting their high dielectricity satisfactorily. Compensating the loss of dielectricity with the use of ferroelectric inorganic compound particles such as barium titanate causes remnant polarization and accordingly makes the gate threshold voltage unstable.
There is another problem in that, if the insulating layer is dried in the low heat treatment step in order to lessen the influence of heat over the resin substrate, the insulating layer is dissolved in an organic solvent in the subsequent step of forming an organic semiconductor layer and an electrode.
As described above, the prior art techniques of forming a gate insulating film at low temperature are incapable of giving satisfactory device performance and the resultant organic semiconductor devices have been found to be undesirable.